The use of carriers and vectors for molecules of interest, especially molecules with a therapeutic effect or markers, has become a major issue for the development of novel diagnostic methods or novel medicines. Specifically, the molecules of interest possess characteristics that influence their pharmacokinetics and biodistribution and which are not always favorable or adaptable to the medium into which they are introduced. These characteristics are, for example, physicochemical properties, such as instability (degradation, a strong tendency toward crystallization . . . ), poor water solubility and/or biological characteristics such as important protein binding, poor physiological barrier bypass, toxicity or biodegradability.
In this context, original nanocarriers were developed from very promising materials never used previously in the biomedical field: porous hybrid organic-inorganic crystalline solids also referred to as Metal-Organic Frameworks (MOFs) (see WO2009/077670, WO2009/077671 and WO2013/178954).
MOFs are crystalline hybrid coordination polymers comprising inorganic units and organic polycomplexant ligands coordinated through ionocovalent bonds to the cations. These materials are organized in three-dimensional frameworks in which the metal clusters are periodically connected together via spacer ligands. These materials are usually porous and are used in many industrial applications such as gas storage, liquid adsorption, liquid or gas separation, catalysis, etc.
Porous hybrid organic-inorganic nanoparticles (nanoMOFs) based on iron carboxylate or zinc imidazolate for instance, have recently been developed to address some of the challenges of galenic. Research in this area started from the statement that there is still a number of active agents with very short plasmatic half-life, difficultly crossing natural barriers of the body, or leading to resistance or toxicity. Nanoencapsulation was proposed as an interesting alternative for the administration of these active agents. Some of these molecules could not be successfully encapsulated in the currently used nanocarriers (liposomes, polymer-based nanoparticles or inorganic-based nanoparticles . . . ). The main reason is the incompatibility of these active molecules, in terms of sufficient interaction to be properly encapsulated, with the materials currently used to develop nanocarriers (such as polymers, lipids or oils). Another reason is the uncontrolled release of the challenging active molecules as a consequence of the fast diffusion from the nanoparticles to the aqueous medium due for instance to their important polarity (such as nucleoside analogues).
NanoMOFs are for example formed from units of iron (III) which produce amphiphilic large cages of defined size (3 to 60 Å) by bridging with endo or exogenous polycarboxylic acid, such as fumaric acid or trimesic acid. It is possible to modulate their pore size, structure and internal microenvironment (hydrophilic/hydrophobic balance) by varying the nature and the functionalization of the carboxylic acids used in the synthesis of nanoMOFs.
Due to their large pore volume and specific surface, nanoparticles or iron carboxylate nanoMOFs proved to be capable of adsorbing, by a simple impregnation in solutions of active principle, very large quantities of such active principle. Especially, they may adsorb more than 40% by weight in the case of some hydrophilic, amphiphilic or hydrophobic molecules. Therapeutic molecules having never before been effectively encapsulated (encapsulated quantities <1 or at most 5% by weight) are thus able to be encapsulated.
The degradability of these nanoMOFs in the body and their biocompatibility was evidenced. For example, injection of doses up to 220 mg/kg did not reveal any signs of toxicity in rats (as assessed by the animal behavior, weight, histology, changes in biological markers, metabolism, biodistribution, elimination). The ability of these nanoMOFs to produce an in vivo magnetic resonance imaging signal (MRI) was also shown. The contrast was attributed to both paramagnetic iron atoms and interconnected channels filled with water, coordinated to the metal sites and/or free. This observation has opened up attractive prospects in theranostic, for monitoring in vivo fate of nanoparticles loaded with active ingredients.
Reference may be made for example to international patent application WO2009/077670 for a description of such MOF nanoparticles.
Methods for modifying the outer surface of nanoMOFs were explored in order to control their interaction with the living environment and enable them selectively addressing in vivo. This is important insofar as the non-modified nanoMOFs are quickly recognized as foreign bodies and are eliminated after a few minutes by the reticuloendothelial system (accumulation in the liver and spleen).
International patent application WO2009/077671 describes methods of surface modification of nanoMOFs. For example, it is currently proposed to adsorb polyethyleneglycol linear chains (PEG) on the surface of nanoMOFs either during their synthesis or post-synthesis. This renders nanoMOFs “stealth”, that is, capable of preventing accumulation in the liver and spleen leading then to longer circulation times.
However, this surface modification strategy has drawbacks, mainly due to the porous nature of MOFs materials, which leads to the adsorption of the surface agent not only on the outer surface of the nanoparticle but also within their porosity and then to a loss of pore volume and surface area. Especially, it was found that the capacity of encapsulation decreased, and there is a greater difficulty in controlling the release of active ingredients encapsulated therein. Thus, there is a need to develop other techniques than adsorption for modifying a porous solid while keeping good encapsulation and release capacities of molecules of interest.
International patent application WO2009/077671 also describes the use of polymers carrying hydrophobic groups capable of interacting with the outer surface of MOFs (such as dextran moieties grafted with fluorescein and biotin) for covering the surface MOFs. However, resulting modified MOF solids lack of stability, especially in physiological medium, which is an obstacle to their in vivo biomedical applications. There is a need to have methods for preparation of modified porous solids keeping a good stability in physiological medium.
International patent application WO2013/178954 describes a further method of surface modification of nanoMOFs. Especially, a surface agent comprising at least one complexing group is used to modify the outer surface of the nanoMOF. The surface agent is linked to the nanoMOF by complexation of metal ions and organic ligands which constitute the nanoMOF outer surface. However, it was found that by this method, the choice of the surface agent, and particularly its dimension in view of the pore size, was an important parameter to consider modifying MOFs in order to keep the porosity of these structures. Indeed, it is required both a surface agent size greater than pore size and functional groups allowing strong interaction between the surface agent and the MOF outer surface. Moreover, by this method, it is not possible to control the polymerization at the MOF outer surface, and as a consequence, the thickness of the polymer coating. Thus, there is a need to have a versatile procedure with regard to the choice of surface agent.
There is thus a need for improvements in terms of functionalization of the outer surface of porous solids, especially MOFs particles. In particular, there is a real need for improved compounds able to evade the immune system and/or rapid capture by certain organs, such as the liver, thus preventing their accumulation in these organs, and capable of vectorizing active ingredients to specific targets. There is a need to develop modified porous solids allowing retaining encapsulation and release capacities, i.e. retaining porosity.
The aim of the present invention is, precisely, to meet these needs and drawbacks of the prior art by providing an outer surface-modified porous solid, especially porous crystalline MOF solid, used as carrier for molecules of interest while keeping a good colloidal stability and a high porosity.
The Applicant unexpectedly evidenced that the Graftfast® method (WO2008/078052) of the invention allows under certain conditions, obtaining modified porous solids. This method enables chemically grafting a polymer at the surface of a solid support. The method is based on chemical reactions, essentially radical reactions of chemisorption and polymerization, hereafter referred to as “copolymerization-like reaction”.
The Graftfast® method may be implemented using adhesion primers as sole building entities and a radical polymerizable monomer.
Adhesions primers are molecules capable of being chemisorbed at the surface of the substrate by radical reaction and allow indirect polymer grafting on any surface type. Generally, the adhesion primer includes diazonium salts which strong reactivity and ensures a robust covalent link between the polymer and the substrate. The reaction of the diazonium salts with a chemical activator having reducing properties allows the reduction of the diazonium and generation of radicals. The activator may be a chemical agent but it may also be a physical condition, such as for example a given temperature or a photoactivation.
The adhesion primer activated under the form of a radical can react either with the surface, forming a primary layer of adhesion, or with a radical polymerizable monomer allowing polymerization initiation. The growing polymer chain in solution then reacts with the radical building layer anchored on the surface.
The Graftfast® method had never been used before to graft polymers on porous substrates.
The Graftfast® method may be implemented using an adhesion primer, in a solvent, in presence of an activator (enabling the formation of radical entities) and in presence of radical polymerizable monomers. The polymer is simultaneously grafted and synthetized directly at the surface of the substrate.
Adhesion primers and monomers used to synthesize and graft the polymer on porous substrates were indeed expected clogging the pores.
Contrary to what was expected, the Applicant surprisingly evidences herein that the grafting of a polymer on a porous surface by Graftfast® method under certain conditions enables retaining porosity.
Thus, this invention relates to the outer surface modification of a porous solid, preferably a MOF solid, by the implementation of specific conditions of the Graftfast® method.